1. Field of the Invention
This invention relates to circuit arrangements. More particularly the invention relates to circuit arrangements for protecting a load connected across an electrical supply against surge voltages, the circuit arrangement incorporating a voltage arrester comprising a gas-filled enclosure housing a pair of electrodes which define between them a discharge gap. In use of such an arrangement the arrester is connected in parallel with the load across the electrical supply lines to the load or across one of the supply lines and an earth line, such that if a surge voltage greater than the breakdown voltage of the discharge gap appears on the supply, the arrester will strike a discharge, thus shortcircuiting the surge voltage and protecting the load.
2. Description of Related Art
As gas-filled excess voltage arresters do not normally strike at voltages of less than about 300 volts, for the protection of, for example, integrated circuits which may be damaged by voltage spikes of only a few tens of volts, it is usual to use a so-called hybrid circuit arrangement, including a second voltage arrester, normally a semiconductor arrester, connected in parallel with the gas-filled voltage arrester, the second arrester being arranged to break down at a somewhat lower voltage than the gas-filled arrester.
A problem arises, however, with known hybrid circuit arrangements where there is a sustained surge voltage which causes the semiconductor arrester to conduct, but is of insufficient magnitude to cause the voltage drop across a resistor and semiconductor arrester to reach the breakdown voltage of the discharge gap of the gas-filled arrester. The gas-filled arrester will then not strike a discharge, and the semiconductor arrester is likely to become overloaded and fail.